Hybrid Mechanical Energy Storage

Summary

The Problem

Renewable energy sources like wind and solar generate electricity intermittently, often producing excess energy when demand is low and falling short when demand peaks. Today, the dominant solution for bridging that gap is electrochemical batteries. While effective, batteries carry significant drawbacks: they rely on resource-intensive mining of lithium, cobalt, and other minerals, degrade over time, pose environmental disposal challenges, and can be prohibitively expensive at scale.

Our Approach

PetrChu is a benchtop-scale hybrid mechanical energy storage system that demonstrates an alternative. The system combines two well-established storage methods into a single integrated platform:

Compressed Air Energy Storage (CAES): Air is compressed into a pressure vessel during periods of excess energy. When power is needed, the stored air drives a reciprocating piston engine connected to an alternator.
Pumped Hydro Storage: Water is pumped to an elevated reservoir, storing gravitational potential energy. During discharge, the water flows through a turbine, also driving the alternator.

One-way clutches connect both prime movers to a shared alternator shaft, allowing either subsystem to generate electricity independently or both to contribute simultaneously. The entire energy path is purely mechanical, no chemical battery is used at any point.

Why It Matters

The global transition to renewable energy demands storage solutions that are scalable, sustainable, and long-lasting. Mechanical storage systems like CAES and pumped hydro already operate at grid scale around the world, but they are typically treated as separate technologies. PetrChu explores what happens when they are combined into a hybrid system with intelligent controls, potentially offering greater flexibility and reliability than either method alone. The people most affected by advances in this space range from utility operators and grid planners seeking alternatives to battery farms, to communities in resource-limited regions where the environmental and economic costs of battery production are felt most directly.

Technical Approach/Methodology

System Architecture

PetrChu approaches the energy storage problem by combining two proven mechanical storage methods into a single benchtop-scale system with a shared power output.

Compressed Air Path: A compressor charges a pressure vessel during periods of excess energy. When power is needed, the stored air is released through a reciprocating piston engine that spins an alternator to generate electricity.
Pumped Hydro Path: A pump lifts water from a lower reservoir to an upper reservoir, storing gravitational potential energy. During discharge, the water flows down through a jet nozzle onto an impulse turbine, which also drives the alternator.
Shared Powertrain: One-way clutches connect both prime movers to the same shaft, allowing either subsystem to generate power independently or both to contribute simultaneously.

Controls and Instrumentation

An embedded control system manages the entire process:

An Arduino microcontroller runs a state machine that sequences charging and discharging operations, monitors sensors for pressure, water level, flow rate, shaft speed, and electrical output, and adjusts valve positions in real time to regulate alternator speed and voltage.
A Raspberry Pi serves as a supervisory computer, logging data and displaying live system performance on an external monitor.

Safety Philosophy

Hardware safety interlocks, including an emergency stop and an arm switch wired directly into the power circuit, ensure the system can shut down safely independent of any software. The 12V control domain and the generated power domain are galvanically isolated from each other. No chemical battery appears anywhere in the energy conversion path, keeping the demonstration purely mechanical.

PetrChu approaches the energy storage problem by combining two proven mechanical storage methods, compressed air and pumped hydro, into a single benchtop-scale system with a shared power output. On the compressed air side, a compressor charges a pressure vessel during periods of excess energy. When power is needed, the stored air is released through a reciprocating piston engine that spins an alternator to generate electricity. On the pumped hydro side, a pump lifts water from a lower reservoir to an upper reservoir, storing gravitational potential energy. During discharge, the water flows down through a turbine, which also drives the alternator. One-way clutches connect both prime movers to the same shaft, allowing either subsystem to generate power independently or both to contribute simultaneously.

An embedded control system manages the entire process. An Arduino microcontroller runs a state machine that sequences charging and discharging operations, monitors sensors for pressure, water level, flow rate, shaft speed, and electrical output, and adjusts valve positions in real time to regulate alternator speed and voltage. A Raspberry Pi serves as a supervisory computer, logging data and displaying live system performance on an external monitor. Hardware safety interlocks, including an emergency stop and an arm switch wired directly into the power circuit, ensure the system can shut down safely independent of any software. All controls, instrumentation, and safety logic are designed so that no chemical battery appears anywhere in the energy conversion path, keeping the demonstration purely mechanical.

Outcomes

Final Deliverables

By the end of this project, PetrChu will deliver a fully functional benchtop-scale prototype demonstrating hybrid mechanical energy storage. The system will accept energy input through a compressor and water pump, store it as compressed air and elevated water, and convert it back to usable electricity — all without a chemical battery in the energy path.

Integrated Mechanical Platform: Both CAES and pumped hydro subsystems mounted on a portable frame with a shared alternator powertrain.
Embedded Control System: Automated state sequencing, real-time voltage regulation, and sensor-driven safety interlocks.
Live Data Dashboard: An external monitor displaying real-time system performance metrics including pressure, flow rate, shaft speed, and electrical output.
Demonstration Panel: A separate visual display showcasing generated electricity using display meters, indicator lights, and resistive loads.

Progress to Date

The project is currently in the design and analysis phase. The team has completed:

System architecture and functional decomposition
Subsystem requirements allocation
Component selection and trade studies
Control system design
Detailed proof-of-concept test plan

Next Steps

In the upcoming build and integration phase, the team will fabricate and assemble the prototype, validate each subsystem independently, and conduct integrated system tests to characterize round-trip efficiency, power output stability, and the system's ability to transition between operating modes. Final documentation will include a comprehensive design report, test results, and this project website as a public-facing record of the work.